Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Topological phases of matter are classified based on symmetries, with nonsymmorphic symmetries like glide reflections and screw rotations being of particular importance in the classification. In contrast to extensively studied glide reflections in real space, introducing space-dependent gauge transformations can lead to momentum-space glide reflection symmetries, which may even change the fundamental domain for topological classifications, e.g., from a torus to a Klein bottle. Here we discover a new class of three-dimensional (3D) higher-order topological insulators, protected by a pair of momentum-space glide reflections. It supports gapless hinge modes, as dictated by Wannier Hamiltonians defined on a Klein bottle manifold, and we introduce two topological invariants to characterize this phase. Our predicted topological hinge modes are experimentally verified in a 3D-printed acoustic crystal, providing direct evidence for 3D higher-order Klein bottle topological insulators. Our results not only showcase the remarkable role of momentum-space glide reflections in topological classifications, but also pave the way for experimentally exploring physical effects arising from momentum-space nonsymmorphic symmetries.

Abstract: Gapless helical edge modes are a hallmark of the quantum spin Hall effect. Protected by time-reversal symmetry, each edge contributes a quantized zero-temperature conductance quantum $G_0 \equiv e^2/h$. However, the experimentally observed conductance in WTe$_2$ decreases below $G_0$ per edge already at edge lengths around 100 nm, even in the absence of explicit time-reversal breaking due to an external field or magnetic impurities. In this work, we show how a time-reversal breaking excitonic condensate with a spin-spiral order that can form in WTe$_2$ leads to the breakdown of conductance quantization. We perform Hartree-Fock calculations to compare time-reversal breaking and preserving excitonic insulators. Using these mean-field models we demonstrate via quantum transport simulations that weak non-magnetic disorder reproduces the edge length scaling of resistance observed in the experiments. We complement this by analysis in the Luttinger liquid picture, shedding additional light on the mechanism behind the quantization breakdown.

Abstract: We report on a detailed investigation of the shell-filling sequence in electrostatically defined bilayer graphene quantum dots (QDs) in the regime of low charge carrier occupation, $N < 12$, by means of magnetotransport spectroscopy. Conductance resonances, so-called Coulomb peaks, appear in groups of four in gate space in good agreement with spin and valley degenerate orbital states in bilayer graphene. Interestingly, an additional bunching into pairs of two is superimposed onto the orbital fourfold degeneracy. We conclude that the additional splitting is caused by electron-electron interaction leading to a renormalization of the QD ground state at half filling of each orbital state. Furthermore, we also report in detail on the influences of the QD geometry on the energy scales of the electron-electron interaction and the impact of the magnetic field on the QD states mainly determined by the QD size-dependent valley $g$-factor.

Abstract: Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is one of the cornerstones for the realization of efficient solar cells and tailored light-emitting devices. We study the effect of compositional disorder on coherent exciton dynamics in a mixed FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ perovskite crystal using photon echo spectroscopy. We reveal that the homogeneous linewidth of excitons can be as narrow as 16$\mu$eV at a temperature of 1.5K. The corresponding exciton coherence time of $T_2=83$ps is exceptionally long being attributed to the localization of excitons due to variation of composition at the scale of ten to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay we conclude that for low-energy excitons, pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Abstract: 2D ferroelectric \{beta}-InSe/graphene heterostructure was fabricated by mechanical exfoliation, and the carrier dynamics crossing the heterostructure interface has been systematically investigated by Raman, photoluminescence and transient absorption measurements. Due to the efficient interfacial photo excited electron transfer and photogating effect from trapped holes, the heterostructure devices demonstrate superior performance with maximum responsivity of 2.12*10e4 A/W, detectivity of 1.73*10e14 Jones and fast response time (241 us) under {\lambda} = 532 nm laser illumination. Furthermore, the photo responses influenced by ferroelectric polarization field are investigated. Our work confirms ferroelectric \{beta}-InSe/graphene heterostructure as an outstanding material platform for sensitive optoelectronic application.

Abstract: I apply the scattering approach within the framework of macroscopic quantum electrodynamics to derive the variances and mean values of the energy density and intensity for a system of an arbitrary object in an arbitrary environment. To evaluate the temporal bunching character of the energy density and intensity, I determine the ratio of their variances with respect to their mean values. I explicitly evaluate these ratios for the cases of vacuum, a half-space in vacuum, and a sphere in vacuum. Eventually, I extend the applicability of this theory to the case of more than one arbitrary object, independent of the geometrical shapes and materials.

Abstract: Phosphorene has emerged as an atomically-thin platform for optoelectronics and nanophotonics due to its excellent nonlinear optical properties and the possibility of actively tuning light-matter interactions through electrical doping. While phosphorene is a two-dimensional semiconductor, plasmon resonances characterized by pronounced anisotropy and strong optical confinement are anticipated to emerge in highly-doped samples. Here we show that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit high tunability in relation to both edge termination and doping charge polarity, and can trigger an intense nonlinear optical response at moderate doping levels. Our explorations are based on a second-principles theoretical framework, employing maximally localized Wannier functions constructed from ab-inito electronic structure calculations, which we introduce here to describe the linear and nonlinear optical response of PNRs on mesoscopic length scales. Atomistic simulations reveal the high tunability of plasmons in doped PNRs at near-infrared frequencies, which can facilitate synergy between electronic band structure and plasmonic field confinement in doped PNRs to drive efficient high-harmonic generation. Our findings establish phosphorene nanoribbons as a versatile atomically-thin material candidate for nonlinear plasmonics.

Abstract: In this work, we present an analytical study on the surface plasmon polaritons in a two dimensional parity anomaly Chern insulator. The connections between the topology in the bulk implied by the BHZ model and the dispersion relations of the surface plasmons have been revealed. Anisotropy has been considered during the calculations of the dispersion relations which allows different permittivities perpendicular to the conductive plane. Two surface plasmon modes each contains two branches of dispersion relations have been found. The topologically non-trivial case gives quite different Hall conductivities compared with the trivial one, which leads to significant modifications of the dispersion curves or even the absence of particular branch of the surface plasmons. Our investigations pave a possible way for the detection of the parity anomaly in a two-dimensional Chern insulator via plasmonic responses.

Abstract: Thermally-induced transitions between bistable magnetic states of magnetic tunnel junctions (MTJ) are of interest for generating random bitstreams and for applications in stochastic computing. An applied field transverse to the easy axis of a perpendicularly magnetized MTJ (pMTJ) can lower the energy barrier ($E_b$) to these transitions leading to faster fluctuations. In this study, we present analytical and numerical calculations of $E_b$ considering both coherent (macrospin) reversal and non-uniform wall-mediated magnetization reversal for a selection of nanodisk diameters and applied fields. Non-uniform reversal processes dominate for larger diameters, and our numerical calculations of $E_b$ using the String method show that the transition state has a sigmoidal magnetization profile. The latter can be described with an analytical expression that depends on only one spatial dimension, parallel to the applied field, which is also the preferred direction of profile motion during reversal. Our results provide nanodisk energy barriers as a function of the transverse field, nanodisk diameter, and material characteristics, which are useful for designing stochastic bitstreams.

Abstract: Phononic crystals (PnCs) are artificially patterned media exhibiting bands of allowed and forbidden zones for phonons. Many emerging applications of PnCs from solid-state simulators to quantum memories could benefit from the on-demand tunability of the phononic band structure. Here, we demonstrate the fabrication of suspended graphene PnCs in which the phononic band structure is controlled by mechanical tension applied electrostatically. We show signatures of a mechanically tunable phononic band gap. The experimental data supported by simulation suggest a phononic band gap at 28$-$33 MHz in equilibrium, which upshifts by 9 MHz under a mechanical tension of 3.1 Nm$^{-1}$. This is an essential step towards tunable phononics paving the way for more experiments on phononic systems based on 2D materials.

Abstract: In this work we present a detailed analysis on the performance of X-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM) as a tool for vector reconstruction of the magnetization. For this, we choose 360$^{\circ}$ domain wall ring structures which form in a synthetic antiferromagnet as our model to conduct the quantitative analysis. We assess how the quality of the results is affected depending on the number of projections that are involved in the reconstruction process, as well as their angular distribution. For this we develop a self-consistent error metric, which indicates that the main factor of improvement comes from selecting the projections evenly spread out in space, over having a larger number of these spanning a smaller angular range. This work thus poses XMCD-PEEM as a powerful tool for vector imaging of complex 3D magnetic structures.

Abstract: We describe both the Fermi velocity and the mass renormalization due to the two-dimensional Coulomb interaction in the presence of a thermal bath. To achieve this, we consider an anisotropic version of pseudo quantum electrodynamics (PQED), within a perturbative approach in the fine-structure constant $\alpha$. Thereafter, we use the so-called imaginary-time formalism for including the thermal bath. In the limit $T\rightarrow 0$, we calculate the renormalized mass $m^R(p)$ and compare this result with the experimental findings for the energy band gap in monolayers of transition metal dichalcogenides, namely, WSe$_2$ and MoS$_2$. In these materials, the quasi-particle excitations behave as a massive Dirac-like particles in the low-energy limit, hence, its mass is related to the energy band gap of the material. In the low-temperature limit $T\ll v_F p $, where $v_F p$ is taken as the Fermi energy, we show that $m^R(p)$ decreases linearly on the temperature, i.e, $m^R(p,T)-m^R(p,T\rightarrow 0)\approx -A_\alpha T +O(T^3)$, where $A_\alpha$ is a positive constant. On the other hand, for the renormalized Fermi velocity, we find that $v^R_F(p,T)-v^R_F(p,T\rightarrow 0)\approx -B_\alpha T^3 +O(T^5)$, where $B_\alpha$ is a positive constant. We also perform numerical tests which confirm our analytical results.

Abstract: We report the presence of exactly and nearly flat bands with non-trivial topology in a three-dimensional lattice model. We first show that an approximate flat band with finite Chern number can be realized in a two-orbital square lattice by tuning the nearest-neighbor and next-nearest-neighbor hopping between the two orbitals. With this, we construct a minimal three-dimensional flat band model without stacking the two-dimensional (2D) layers. Specifically, we demonstrate that a genuine three dimensional non-trivial insulating phase can be realized by allowing only nearest and next-nearest hopping among different orbitals in the third direction. We find both perfect and nearly perfect flat bands in all three planes at some special parameter values. While nearly flat bands carries a finite Chern number, the perfect flat band carries zero Chern number. Further, we show that such a three dimensional (3D) insulators with flat bands carry an additional three dimensional topological invariant, namely Hopf invariant. Finally, we show that a 3D construction of lattice model with Hopf invariant from a 2D Chern insulator is model specific and appearance of flat bands is not guaranteed in the Hopf-Chern system with only nearest and next-nearest hopping among distinct orbitals.